Significance Statement

Carbon-related materials are expected to play more and more important roles in this environmentally conscious society as carbon is a light, strongly binding, ubiquitous and environment-friendly element. Among them, carbon nanomaterials are very attractive as the single element can form a variety of structures due to its flexible sp2 framework and their properties could be superior or peculiar depending on their structures as one can see it in fullerene, carbon nanotube and graphene. We reported two main achievements in this paper, including the development of a new synthesis method for carbon nanomaterials and creation of a new carbon nanomaterial with a novel and functional nanostructure using this synthesis method. The non-equilibrium reaction fields, such as high temperature gradient, rapid heating or cooling, and so on, are known to be very effective to produce metastable nanostructures. In this study, we have developed a facile technique for chemical vapor deposition (CVD), named submarine-style CVD method, in which a CVD chamber with an open bottom (Figure, left top panel) is immersed in organic liquid, to keep the walls and the inside of the chamber cold and dry, respectively, and to supply vaporized carbon source to catalysts directly. We have succeeded to synthesize single-walled carbon nanotube, double-walled carbon nanotube, carbon nanowall, and carbon nanoribbon as well as the new carbon nanomaterial, named carbon nanopot, using this technique.

We have produced carbon nanopot and revealed its peculiar nanostructure through FE-SEM and TEM observations, XPS, EMPA, micro-Raman spectroscopy, and proposed a new growth model to account for the formation of the peculiar nanostructure in this study. Our findings are as follows, (1) the typical size of carbon nanopot is 20-40 nm in outer diameter and 100-200 nm in length and it has a deep hollow inner space (aspect ratio of ~10) with an open end and a closed end (Figure, right panel); (2) carbon nanopot has a complex and regular nanostructure in which graphene edges are distributed unevenly at its outer surface and exposed densely around its closed end (Figure, inset in the right panel); (3) the graphene edges are presumably hydroxylated (Figure, schematic diagram in the left bottom panel); (4) carbon nanopot is synthesized in series with the appearance of fiber (Figure, left bottom panel); (5) the length of carbon nanopot fiber is 20-100 mm typically; (6) neighboring carbon nanopots do not share their graphene layers and each nanopot is separable from the fiber. Note that negative curvature, which is expected to induce magnetic moments theoretically, was observed clearly in some nanopots (Figure, right panel, between MWNT section and Expanding hollow neck). These features would be favorable to the application of carbon nanopot to drug delivery, functional composite materials, sensing materials, or electrode materials of high performance batteries. The densely localized graphene edges might enable one to develop new nanomaterials processing. The formation mechanism of the unique structure was also a challenging issue. We paid attention to the fact that choice of graphene oxide as catalyst support was as essential to the formation of carbon nanopot as the choice of the submarine-style CVD method. A tag of war over a catalyst particle between the sp2 surface of graphene oxide and the inner sp2 surface of carbon nanopot could be responsible for the repeated formation of the peculiarly nanostructured material.

To summarize, carbon nanopot is a promising newcomer in the family of carbon nanomaterials. Unlike conventional nanomaterials, carbon nanopot has a complex and regular nanostructure with a deep hollow inner space, densely localized graphene edges, presumably hydroxylated, around its closed end, and negative curvature. We expect carbon nanopot will exhibit distinguished properties in the application to drug delivery, functional composite materials, sensing materials, electrode materials, etc. and will make it possible to develop new nanomaterials processing. The proposed mechanism for formation of nanopot could be a suggestion for development of carbon nanomaterials with exotic structures. The submarine-style CVD method, a facile technique to generate a non-equilibrium reaction field, has been proved quite effective in the development of carbon nanomaterials.

Figure Legend: (Left top) Schematic of the synthesis chamber of the submarine-style CVD apparatus. The front glass plate of the chamber is shown detached for ease of observing the inside. (a) Electrodes, (b) borosilicate glass plates, (c) catalyst-loaded silicon substrate, (d) substrate support, and (e) carbon plate heater. (Left bottom) TEM image on carbon nanopot fibers. The arrow points to the open end of a carbon nanopot. A schematic diagram of a carbon nanopot fiber hydroxylated at the graphene edges around their closed ends is also exhibited (not to scale). (Right) TEM image of carbon nanopots. Carbon nanopot consists of several parts with different structural features as labeled. The inset is a magnified view of the section bordered by the dashed box in the main image. The value indicates the average distance between layers. The arrows point to the graphene edges along the outside of the tapering tube part. After H. Yokoi et al., J. Mater. Res. 31, 117 (2016). (Copyright: Materials Research Society, Reprinted with permission).

We have developed a new synthesis method that includes a chemical vapor deposition process in a chamber settled in organic liquid, and applied its nonequilibrium reaction field to the development of novel carbon nanomaterials. In the synthesis at 1110-1120 K, using graphene oxide as a catalyst support, iron acetate and cobalt acetate as catalyst precursors, and 2-propanol as a carbon source as well as the organic liquid, we succeeded to create carbon nanofiber composed of novel pot-shaped units, named carbon nanopot. Carbon nanopot has a complex and regular nanostructure consisting of several parts made of different layer numbers of graphene and a deep hollow space. Dense graphene edges, hydroxylated presumably, are localized around its closed end. The typical size of carbon nanopot was 20-40 nm in outer diameter, 5-30 nm in inner diameter and 100-200 nm in length. A growth model of carbon nanopot and its applications are proposed.